14

This is a pretty mathematical question. Given that:

  • vegan diet has a greatly reduced environmental impact due to the smaller amount of greenhouse gases emissions (useful links: 1 2)

  • world population is increasing over time

How many people are needed (in %) to switch to a vegan diet in order to reach a decrease of GHG emissions of 10% from the 1990 level? Calculations are ok for any country, but it would be quite interesting to have them for United States or for some european country.

(Note: if I missed some relevant data, just spot it and find it on the web)

  • Does the Kyoto protocol actually aim for a 10% GHG emission decrease from the 1990 level? – cellu Jun 10 '17 at 21:42
  • Each annex I country set it's own target making it difficult to talk about global targets. The 10% figure may be a country level goal. – nloewen Jun 14 '17 at 12:44
10

TLDR: The short answer is that it's not possible. Greenhouse gas emissions come from a diverse set of sources, and only the weakest targets can be met by focusing solely on one area. Even if every person on earth became a vegan, that would not be enough, although it would help significantly.


I'm going to start with a major simplifying assumptions to ease calculation. Any greenhouse gas emissions attributed to agriculture would alleviated with a vegan diet. With this assumption, a 100% vegan world results in zero greenhouse gas emissions due to agriculture. This is obviously wrong, but will give us an upper bound for how much diet changes can affect total greenhouse gas emissions.

US

In the US in 2015, agriculture was responsible for 9% of greenhouse gas emissions.1

Total U.U. Greenhouse Gas Emissions by Economic Sector in 2015

Assuming veganism can eliminate 100% of agricultural emissions, if every person living in the United States transitioned to a vegan diet, that would not be enough to achieve an emissions decrease of 10% over 2015 levels, nevermind 1990.

Global

Globally, all of agriculture, forestry, and other land use accounts for 24% of greenhouse gas emissions.2, 3

Global Greenhouse Gas Emissions by Economic Sector

Warming projections which keep the probability of staying below 2°C of warming by 2100 "about as likely as not" require emissions to be reduced by at least 25% by 2050 at the low end of the range.3

Key characteristics of the scenarios collected and assessed for WGIII AR5

Even in optimistic scenarios, the elimination of all agriculture, forestry, and other land use emissions globally are insufficient, thus we can easily say that a vegan diet alone will not be able to reach our emission targets.

  • I'm a bit concerned with the first pie chart. What exactly is accounted, e.g., in the emissions from agriculture or industry? I think farming and industry are responsible for consuming the largest share of the electricity production. Furthermore, products such as cow milk require more pasteurizing and processing than plant milk so vegan diets would affect the industry slice of the pie. Plant milk ingredients can be dried, unlike raw milk which needs constant cooling, so it's much easier and ecological to transport, so vegan diets will reduce the size of the transportation slice. – ecc Jun 14 '17 at 7:20
  • I know you grossly overestimated when saying vegans would bring the agriculture emissions to zero, but I really wonder if it would not reduce it even more than that. Although the end result is still likely that no amoung of vegans would reach the Kyoto protocol target. – ecc Jun 14 '17 at 7:21
  • The agriculture emissions in the US chart include agricultural soil management, enteric fermentation, manure management, rice cultivation, urea fertilization, liming, and field burning of agricultural residues. It would be very interesting to have numbers on indirect agricultural emissions. – nloewen Jun 14 '17 at 13:04
  • I also ignored Kyoto targets for the most part since only annex I countries have targets and the US doesn't have one. I used the unsourced 10% figure in the question for the US target, and 2% warming as the global target. – nloewen Jun 14 '17 at 13:08
4

In 2006, the UN published the study Livestock's Long Shadow and stated, that the livestock sector accounts for 18% of the greenhouse gas emissions. As stated in the source cited by the questioner, there is an Oxford University study on the Dietary greenhouse gas emissions of meat-eaters, fish-eaters, vegetarians and vegans in the UK concluding that "an average 2,000 kcal high meat diet had 2.5 times as many GHG emissions than an average 2,000 kcal vegan diet." Naively taking these numbers as a basis, the global GHG emissions would drop by 10.8% if everyone became vegan:

 100
- 18       [livestock]
+ 18 / 2.5 [equivalent vegan diet]
= 89.2

100% - 89.2% = 10.8%

The questioner asked for a "pretty mathematical" approach. Of course the potential of climate change mitigation through dietary change exceeds the direct emission savings. Especially establishing carbon sinks on the released land (livestock accounts for 80% of total anthropogenic land use), e.g. through forestation or the introduction of swamps, can additionally extract GHG from the atmosphere.

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